Much work has been done and is reported in the prior art on processes for making linear and cyclic phosphonitrilic chloride oligomers from ammonia and ammonium derivatives and phosphorus chlorides.
Methods for the preparation of the cyclic species especially the trimer and tetramer are the focal point of recent investigation. Representative prior art processes are described, for example in U.S. Pat. No. 3,359,080 issued Dec. 19, 1967 to Ridgeway et al wherein elemental chlorine, PCl.sub.3 and NH.sub.4 Cl are reacted in an inert solvent; U.S. Pat. No. 3,367,750 issued Feb. 6, 1968 to Jaszka et al wherein use of ammonium chloride of ultrafine particle size (e.g., less than 5 microns) with phosphorus pentachloride is taught to produce a higher percentage of cyclic oligomers in the reaction product than larger particles of ammonium chloride; U.S. Pat. No. 3,462,247 issued Aug. 19, 1969 to Paddock et al wherein a metallic salt is used to catalyze the PCl.sub.5 -NH.sub.4 Cl reaction; U.S. Pat. No. 3,656,916 to Schiedermier et al issued Apr. 18, 1972 wherein PCl.sub.5 and NH.sub.3 are reacted in an inert solvent, the ammonia being added under controlled rates of addition, and U.S. Pat. No. 3,780,162 issued Dec. 18, 1973 to Bergeron et al wherein ammonia and PCl.sub.5 are reacted under pressure in the presence of hydrogen chloride. Belgium Pat. No. 845,952 published Mar. 8, 1977 discloses a method for the preparation of cyclic oligomers by simultaneously introducing phosphorus trichloride, chlorine and ammonia into a stirred solvent such that the rate of phosphorus trichloride addition does not exceed the stoichiometric ratio with respect to the ammonia and chlorine introduced.
Much early work concerns preparation of linear oligomers, for example by the reaction of phosphorus pentachloride and ammonium chloride, and is described by Allcock in his book Phosphorus-Nitrogen Compounds, Academic Press, New York (1972), 110 et. seq. See also M. Becke-Goehring and E. Fluck, Agnew. Chem., 74, 382 (1962), and Paddock et al, Advances in Inorganic Chemistry and Radiochemistry, Academic Press, New York (1959), Volume 1, 351-53, 359-601. These processes generally require reacting solid ammonium chloride and solid phosphorus pentachloride in certain proportions in a suitable solvent and for a relatively long time, for example 30 to 40 hours or more. Accordingly there still remains a need for a process which enables the preparation of low molecular weight phosphonitrilic halide oligomers in high yield within a relatively short time. Such a process is provided by the present invention.